librnd 3.1.0
~~~~~~~~~~~~

3.1.0 implements all new API that was pending since 3.0.0:
 - install more src_3rd minilibs (gensexpr, all headers for genht)
 - more efficient safe_fs stat() call
 - DAD subdialog widget
 - DAD string region selection (used by the file selection dialog)
 - conf diagnostic actions
 - generalize preferences dialog; last 4 tabs of the pcb-rnd preferences dialog
   moved to librnd among with an app-tab extensible preference dialog framework

Besides the new API, there are a lot of invisible, internal change
for a gtk refactoring to enable gtk4 within a few releases:
 - lib_gtk_common is split to gtk2 and gtk4 variant
 - some of the gtk opengl rendering is refactored for reuse with gtk4
 - opengl rendering code changes to widen the range of suitable opengl versions

A new, gtk HID config node is introduced to make it possible to scale icons up
(useful on high resolution screen).