[pvr] adjust labels for setting pvrplayback.startlast

This change the current used labels 'Yes' and 'Minimised' to 'Foreground' and 'Background', so it's easier to understand what's happen if you choose one of these options.
[pvr] remove unnecessary filler method for setting 'pvrplayback.startlast'

This removes the filler method for setting 'pvrplayback.startlast' and add options directly to the settings.xml.
[pvr] refactor: change method parameter bPreview to bMinimised to be more reasonable
[pvr] refactor enum ChannelStartLast to reflect the setting
[videodb] fix seasons update query for mysql, which doesn't like SELECT subqueries on the same table
[cleanup] followup to f86b9d2884
[videodb] fix tvshow view for mysql by utilising a subview. Thanks to @sccplg
Merge pull request #4969 from xhaggi/pvr-refactor-setting-pvrplayback-startlast

[pvr] adjust labels and refactoring for setting 'pvrplayback.startlast'
Merge pull request #4988 from mkortstiege/threadmsgs

[cleanup] followup to f86b9d2884
Merge pull request #4985 from jmarshallnz/fix_mysql_query

[videodb] fix seasons update query for mysql
[appmessenger] fix MediaPlay regression introduced by f86b9d2
Merge pull request #4993 from mkortstiege/playfile

[appmessenger] fix playfile regression introduced by f86b9d2
[stdstring] get rid of CStdString from utils/ (part 1)
[stdstring] get rid of CStdString in dbwrappers/
[stdstring] get rid of CStdString in playlists/
[stdstring] get rid of CStdString in view/
[stdstring] get rid of CStdString in network/
[stdstring] get rid of CStdString in filesystem/*File.cpp|h
[stdstring] get rid of CStdString in filesystem/*Directory.cpp|h
[stdstring] get rid of CStdString in filesystem/(Music|Video)DatabaseDirectory/