Simplifying QgsOgcUtils

After commit 7d66529d7884b73070dbce80b1d4060d841a6933 Add GML3 to
QgsOgcUtils, I have to simplify this class to only have :
* geometryFromGML : read GML2 or GML3
* geometryToGML : create GML2 (as default) or GML3
[FEATURE] attribute editing extension
- allow resetting of fields to NULL by button
- use user defined date formats in forms, identify results and attribute table
- add support for date type in postgres provider
OGR typedefs do not seem to be necessary
restore "stretch using current extent" functionality in layer context
menu (fix #7070)
update Random and Regular points tools to API changes (fix #7350)
Identify GetFeatureInfo Url context menu fix
update Vector Grid to vector API changes (fix #7322)
Prevent overshoots with line offsets
save used contrast enhancement algorithm in private class member (follow up 5ecb26810a)
Set combo box maximum for precision <= type length
fix Export geometry info tool (fix #7354)
[FEATURE] brightness and contrast filter for rasters
[API] add read/writeXML methods to QgsRasterInterface
add icons for new raster tools (thanks, Robert!)
fix QgsRasterInterface sip bindings
fix #7357
hide clear button in filter line edit, when widget is disabled
Moved creation of expression from OGC filter to QgsOgcUtils
Moved visitor-based expression -> OGC filter code to QgsOgcUtils
Moved expression to OGC filter code to QgsOgcUtils