[FEATURE][composer] Allow control of hoz and vert margins for labels

Previously only a single margin setting would apply to both
horizontal and vertical margins. This change allows users to specify
different horizontal and vertical margins.
[FEATURE][composer] Allow negative margins for label items

If negative margins are set for a label, the label contents will
begin outside the bounds of the label. This is desirable for
aligning label items with other items while allowing for optical
margin alignment for the label type.
[FEATURE][composer] Add view menu option to hide bounding boxes

This allows users to hide the bounding boxes for selected items
within a composition. It's a handy feature for allowing interaction
with items while previewing exactly how they will look when the
composition is exported, without large boxes blocking the view.
[FEATURE][composer] Full screen mode for composer

This change adds a full screen mode for the composer. Full screen
mode can be enabled via a shortcut key (F11) or via a menu action.
[FEATURE][composer] New action for toggling visibility of panels

This change adds a new action to the composer which hides or shows
all panels from the window. It's useful for maximising the space
available to interact with a composition. A shortcut (F10) and menu
item can be used to toggle this option, or via pressing 'tab' while
the main composer view is focused.
add link to 2.6 doxymentation to master version
[processing] add Regular points algorithm (addresses #5953)
[processing] add Symetrical difference algorithm (addresses #5953)
[processing] fix Join by location alg
[processing] add Vector split algorithm (addresses #5953)
[processing] add Vector grid algorithm (addresses #5953)
fix directory layout