[FEATURE] Configure custom environment variables

- Fix for #3097
- Could help plugins, like SEXTANTE, work better across platforms via abstracted coding based upon env vars
- In app prefs, configure variables and view current set, with tooltips to show pre-startup value (if any)
Add checkbox to filter non-QGIS-specific vars from current env vars table
[API] make primary key field indexes available:
- add QgsVectorLayer::pendingPkAttributeList()
- add QgsVectorDataProvider::pkAttributeIndexes()
- skip primary key attribute when pasting features (fixes #6164)
- default to skip primary key attribute in when merging features/attributes
- reset primary key attributes to default value when splitting features (better fix for #6060)
- comment out unused QgsPasteTransformations
Switch main Options dialog to vertical tabs

- Icon-only mode enabled by fully moving splitter to left
attribute table: NULL values are considered less than other values (fixes #6715)
fix #6726
doxygen warnings fixes
fix variant type detection for bool, int and double values
remove separator
indentation update