Adding enforcement of the wanted graphics system GL/GLES
- GRAPHICS=GL/GLES
revert partial changes for  690324e. Removel of
`convert_distance_to_scroll` was unintended.
closes #1896
increase scrollbar width
Merge pull request #1897 from voen/patch-2

Make property list draggable by scrollbar too
doc: add instructions for Raspberry Pi
- properly unset the selected_node attribute, which addresses issue #1898
- addresses issue #1839
doc: update gettingstarted image to add raspberry pi
Merge pull request #1902 from aspidites/showcase_crash_fix

- addresses issue #1839
doc: grammar correction in uix/widget.py
fix: added blank line to fix doc build
doc: changed to standard version tagging
doc: restored python highlighting to remove red error surrounds
- promoted exit_on_escape functionality to BaseWindow and implemented a
  callback for it
- PEP8 fixes
doc: layout/tag fixes to uix/gridlayout.py
Correct label padding to be positive, not negative.
prelim focus behavior.
doc: fixed version changed spacing
doc: corrected versionchanged spacing + small grammar corrections