Fix for #7289, add CMake option to enable building with Qt jom (nmake) on Windows

- Can now compile using NMake with jom directly from QtCreator using MSVC compiler (2008) and OSGeo4W devel libs
- Make app capable of running from build directory
- Caveat: may be better way to do it; could have better warning control
Revert unnecessary disabling of unknown warnings
fix #7386
replace some message box with message bar in main app
Merge pull request #468 from slarosa/master

replace some message box with message bar in main app
More coherent version string handling in qgis server
Merge pull request #464 from dakcarto/nmake-with-jom_1

Fix for #7289, add CMake option to enable building with Qt jom (nmake) on Windows
Disable 'overloaded virtual' warning globally when using clang

- Commit 1145bd2 now causing 200+ 'overloaded virtual' warnings
composer legend: draw single symbol layers as single line
[sextante] fixed problem in fix for #7348
Do not load metadata from __init__.py
Remove the support for reading python plugin metadata from __init__.py

From now on, plugins must have metadata.txt file (with INI format)
that is used for storing of metadata. This has two advantages:
- plugins do not need to be loaded in order to read metadata
- repository can parse the metadata more easily
translation update: khmer by Khoem