gdb/tui: add 'set tui mouse-events off' to restore mouse selection

Rationale:
I use the mouse with my terminal to select and copy text. In gdb, I use
the mouse to select a function name to set a breakpoint, or a variable
name to print, for example.

When gdb is compiled with ncurses mouse support, gdb's TUI mode
intercepts mouse events. Left-clicking and dragging, which would
normally select text, seems to do nothing. This means I cannot select
text using my mouse anymore. This makes it harder to set breakpoints,
print variables, etc.

Solution:
I tried to fix this issue by editing the 'mousemask' call to only enable
buttons 4 and 5. However, this still caused my terminal (gnome-terminal)
to not allow text to be selected. The only way I could make it work is
by calling 'mousemask (0, NULL);'. But doing so disables the mouse code
entirely, which other people might want.

I therefore decided to make a setting in gdb called 'tui mouse-events'.
If enabled (the default), the behavior is as it is now: terminal mouse
events are given to gdb, disabling the terminal's default behavior.
If disabled (opt-in), the behavior is as it was before the year 2020:
terminal mouse events are not given to gdb, therefore the mouse can be
used to select and copy text.

Notes:
I am not attached to the setting name or its description. Feel free to
suggest better wording.

Testing:
I tested this change in gnome-terminal by performing the following steps
manually:

1. Run: gdb --args ./myprogram
2. Enable TUI: press ctrl-x ctrl-a
3. Click and drag text with the mouse. Observe no selection.
4. Input: set tui mouse-events off
5. Click and drag text with the mouse. Observe that selection works now.
6. Input: set tui mouse-events on.
7. Click and drag text with the mouse. Observe no selection.

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