small improvements to album/artist menus

- tweaks to the way the number of columns and the size of the picture is selected
- try to make long titles use less width
- when loading pictures in the background, prioritize currently displayed ones
improvements to the Breakdown_List function

this function is used to split long menus into submenus, its most common use is for the artist selection menu
- lots of changes and tweaks to the algorithm used for splitting
- now takes into account the size of the screen
- it can now split into 2 levels of submenu for really long lists
- makes the menu appear faster by creating the submenus at the last moment
fix behavior of some hidden album/song submenus

In many album/artist menus right-clicking on an item change the artist/album item into an album/song submenu, in some places the action when selecting an item of these hidden submenus was not right
masstagging dialog: change the artist/... combobox

The list of all artist/album/... values in the combobox is replaced by a button :
- speed-up the creation of the masstagging dialog
- makes which values are in the selected songs clearer
add ChooseSongFromAlbum command, meant to be used in layouts
fix arist/album menu not working properly when using 2-level of submenus
update Czech translation
makefile : add checkpo target to check format strings in .po files
remove disc/track/year tag rather than writing 0 in it
layouts: make it possible for boxes (at least WB) to use hover_* options
add Japanese translation
shimmer layout: make 3 strings translatable
fix some harmless warnings
update Korean translation
add Serbian translation
update albuminfo plugin to work with current allmusic.com
update Portuguese translation
fix double-click on cloud/mosaic view when nothing is selected
remove a deprecated option in some contributed layouts
add partial Finnish translation