New GetSizeFromTextSize()

[mmarsan]

This is the first part of a sequence of patches I'll submit.

The "part_1" implements main public GetSizeFromTextSize() at wxControl and its needed DoGetSizeFromTextSize() for wxTextCtrl, wxChoice, and wxComboBox wxGTK Gtk+{2,3} versions.

I've modified 'combo', 'controls', and 'text' samples in order to see this new feature. I've changed sizers flags so they don't hide real sizes.

Next planned parts:
wxMSW version of part_1
wxComboCtrl, wxSearchCtrl, wxSpinCtrl implementations

[vadz]

Thanks, I'll apply this soon (with some renamings and docs for the new function).

[mmarsan]

I was reserving docs for the time all other parts were submited. If you do this job, I can't object anything ;)

Before I send more parts, I'll wait for you to apply it, so I can see the new names.

[VZ]

(In [72935]) Add wxControl::GetSizeFromTextSize() to size the control to its text.

This function can be used to size a, say, wxTextCtrl to be exactly of the size
needed to enter the given amount of text in it.

This patch adds wxGTK implementation for wxTextCtrl, wxChoice and wxCombobox;
changes to the samples and the documentation.

Closes #14812.