settings: move <dependencies> tag out of <control> tag
settings.xml: remove unneeded <control> tags for action settings
settings: move <allowempty> tag into <constraints> tag
settings: move <addontype> tag into <constraints> tag
settings: move <heading> tag into <control> tag
settings: move <formatlabel> tag into <control> tag
settings: use #define for XML tag parsing
Merge pull request #2763 from Montellese/settings_definition_cleanup

settings: cleanup of the XML definition syntax