Update and condense wxGTK README

Remove obsolete information and parts already documented elsewhere.

Also remove the part about wxWidgets crashing your harddisk or
destroying your monitor because I'm a killjoy.

docs/gtk/readme.txt

@@ -1,35 +1,25 @@
-   Welcome to wxWidgets for GTK+
-   -----------------------------
+   Welcome to wxWidgets for GTK
+   ----------------------------
 
-You have downloaded the GTK+ port of the wxWidgets GUI library.
+You have downloaded the GTK port of the wxWidgets GUI library.
 
-It is required to use at least GTK+ 2.6 and some features are
-only available when using GTK+ 2.10 or higher. Note that even if
-you compile wxWidgets against a later GTK+ version, your application
-will still work with older GTK+ version down to version 2.6 as wxWidgets
-tests for the presence of newer features at run-time and it
-will fall back to a generic implementation if these features
-are not present in the GTK+ version used.
+This port works best with GTK 3, but also supports versions as
+old as GTK 2.6 (although some features are only available when
+using GTK+ 2.10 or higher). Note that even if you compile
+wxWidgets against a later GTK version, your application will
+still work with older GTK versions with the same major version
+as wxWidgets tests for the presence of newer features at run-time
+and it will fall back to a generic implementation if these
+features are not present in the GTK version used.
 
-wxWidgets 2.9.4 adds support for GTK+ 3, which can be enabled
-with configure option --with-gtk=3.
-
-wxWidgets 2.8 still supports GTK+ 1.2, but no development
-is done on that port anymore and support for GTK+ 1.2 will
-probably be dropped entirely. Indeed, one of the major steps
-in the development cycle leading up to wxWidgets 2.8 was
-that the codebase for GTK+ 1.2 and GTK+ 2.0 were separated
-so that improvements and clean-ups to the GTK+ 2.0 port
-could be done without caring for GTK+ 1.2.
-
-More info about the wxWidgets project (including all the
+More information about the wxWidgets project (including all the
 other ports and version of wxWidgets) can be found at the
 main wxWidgets homepage at:
 
     https://www.wxwidgets.org/
 
 Information on how to install can be found in the file
-INSTALL.txt, but if you cannot wait, this should work on
+install.md, but if you cannot wait, this should work on
 many systems:
 
     mkdir build_gtk
@@ -40,10 +30,8 @@ many systems:
     make install
     ldconfig
 
-In order to use wxGTK with GTK 1.2 you need to use --with-gtk=1
-configure option. To build the library in Unicode mode (all
-strings will be wide strings and so on) you need to configure
-with GTK 2.0 and add --enable-unicode.
+In order to use wxGTK with GTK 2 you need to use --with-gtk=2
+configure option.
 
 Please note that it is strongly advised to build the library in
 a separate directory, as shown above, as it allows you to have
@@ -51,21 +39,8 @@ different builds of it (for example with and without debug) in
 parallel. If you build in the source directory, don't forget to
 do "make clean" before changing the configuration!
 
-When you run into problems, please read the INSTALL.txt and
-follow those instructions. If you still don't have any success,
-please send a bug report to one of our mailing list, INCLUDING
-A DESCRIPTION OF YOUR SYSTEM AND YOUR PROBLEM, SUCH AS YOUR
-VERSION OF GTK, WXGTK, WHAT DISTRIBUTION YOU USE AND WHAT ERROR
-WAS REPORTED. I know this has no effect, but I tried...
-
 Please send problems concerning installation, feature requests,
 bug reports or comments to the wxWidgets users list. These can
 be found at https://www.wxwidgets.org/support/mailing-lists/
 
-wxWidgets doesn't come with any guarantee whatsoever. It
-might crash your harddisk or destroy your monitor. It doesn't
-claim to be suitable for any special or general purpose.
-
-  Regards,
-
-    Robert Roebling
+Please see the manual for further information.