Const correctness for numerous data provider methods

[nyalldawson]

This PR marks numerous QgsDataProvider, QgsVectorDataProvider and QgsRasterLayer methods correctly as const.

It partly addresses qgis/qgis4.0_api#48

Note that many individual data provider members have been marked as mutable as a consequence - I think this is acceptable, as it's semantically correct. These methods should be marked const, and it's valid to make any cached variables which are altered as a result mutable.

[nyalldawson]

I should add:

• this has no consequence on PyQGIS code, and it's only a break for 3rd party data providers
• the biggest advantage is that core code which calls these methods can be const corrected as a result, avoiding the need for some const casting manipulation
• non const getters look very wrong to me, so this helps me sleep better at night ;)

[nyalldawson]

Any objections if I merge?

[nyalldawson]

Manually pushed as 1bafa80