Fix behaviour of wxTextCtrl without wxTE_PROCESS_TAB in wxGTK

TAB should be used for navigation by default and only should be inserted
into the control as a literal character if wxTE_PROCESS_TAB is specified
for consistency with wxMSW and because this behaviour is much more
useful by default.

Fix this by calling gtk_text_view_set_accepts_tab() as appropriate for
multiline text controls. For single line ones, the behaviour is
unchanged but it's more reasonable as TAB is always handled as if
wxTE_PROCESS_TAB were not specified and it doesn't seem really useful to
try to support wxTE_PROCESS_TAB for them anyhow, so just document this
limitation.

Also remove the outdated/misleading documentation of this style, notably
don't say that it is required to get char events for TAB presses as
these events are generated both with and without this style in both
wxGTK and wxMSW.

Closes https://github.com/wxWidgets/wxWidgets/pull/704

interface/wx/textctrl.h

@@ -945,10 +945,12 @@ public:
            (otherwise pressing Enter key is either processed internally by the
            control or used to activate the default button of the dialog, if any).
     @style{wxTE_PROCESS_TAB}
-           The control will receive @c wxEVT_CHAR events for TAB pressed -
-           normally, TAB is used for passing to the next control in a dialog
-           instead. For the control created with this style, you can still use
-           Ctrl-Enter to pass to the next control from the keyboard.
+           Normally, TAB key is used for keyboard navigation and pressing it in
+           a control switches focus to the next one. With this style, this
+           won't happen and if the TAB is not otherwise processed (e.g. by @c
+           wxEVT_CHAR event handler), a literal TAB character is inserted into
+           the control. Notice that this style has no effect for single-line
+           text controls when using wxGTK.
     @style{wxTE_MULTILINE}
            The text control allows multiple lines. If this style is not
            specified, line break characters should not be used in the controls