webserver: cosmetic refactoring
webserver: add support for If-Unmodified-Since HTTP header
webserver: add Cache-Control HTTP header to cacheable responses
webserver: handle Cache-Control: no-cache and Pragma: no-cache
webserver: use defines for HTTP header fields and values
webserver: silence warning
webserver: refactor IsAuthenticated()
[Confluence] remove addon ratings
FIX: [aml;pivos] Fix 1080 on MX
Merge pull request #5987 from koying/fixaml1080
Merge pull request #5573 from Montellese/webserver_caching

webserver: improved caching control
[art] fix missing container art for album paths
[Fix] Fix a memory leak in Setting.cpp.
[keyboard] only use English names for layouts
[Fix] Fix a memory leak in PlayerSelectionRule.cpp.
Merge pull request #5989 from mkortstiege/container_albumart

[art] fix missing container art for album paths
Merge pull request #5986 from ronie/confluence-addon-ratings

[Confluence] remove addon ratings
Merge pull request #5974 from ace20022/mem_leaks

[Fix] Fix two memory leaks
[dbwrappers] have query() take a std::string, we copy direct into a std::string anyway in the functions
[smartplaylist] adds routine for generating queries against videodb link tables