Make tab label length and window title length configurable

[00AR]

Implements #3344
Added a GtkSpinButton in "Notebook tabs" in Interface preferences. This will take as input, number of characters that the user wants to display on the tab label.
For now, Maximum value that can be entered: 100000
Default value: 99999(as suggested by @elextr to be very big number)
Minimum value that can be entered: 3
The value is stored in the variable tab_label_len declared in GeanyInterfacePrefs struct. This value will be written in geany.conf when save_dialog_prefs of keyfile.c is executed.
The tab label will get updated dynamically on clicking the Apply/Ok button in the Preferences dialog.

[eht16]

Apart from the few comments, it looks very good.

By accident I found #2112 which would be solved by this as well.

Maybe you could add "closes #2112 and #3344" to the commit message and rephrase it a bit as "display size" might be confused with the size of the monitor. Maybe something like "Make tab label length configurable".

[00AR]

I missed this, does the documentation also needs to be updated?

[elextr]

does the documentation also needs to be updated?

Yes, just add the description in geany.txt where that dialog is described, @eht16 will add the dialog to his list of images to be updated before release ;-)

[eht16]

Thanks for the changes, @00AR.

LGTM. I will merge in a few days if there will be no objections.

[00AR]

LGTM. I will merge in a few days if there will be no objections.

Alright. Thanks!