[cleanup] drop unused code blocks in contextmenu from ancient skins.

Skins for 14.X should use a single background image (id 999) with border.
and must have a grouplist with id 996 for the context menu.
[cosmetic] change name of define to better reflect what it's for (old python guilib stuff)
[guilib] remove backward compatibility of converting edit controls to buttons.

For v14 ensure the following controls are edit controls:

SmartPlaylistEditor.xml:
 id 12, name of smart playlist

SmartPlaylistRule.xml:
 id 17, value of rule

DialogMediaSource.xml:
 id 12, name of media source

DialogNetworkSetup.xml:
 id 11, server address
 id 13, port
 id 14, username
 id 15, password
 id 16, remote path
[addons] adds GetDependencyVersion() to CAddon - used in place of GetXbmcApiVersionDependency
[guilib] adds APIVersion() to CSkin to determine which API version the skin wants.
[guilib] drop the separate default for the aspect ratio inside containers. It previously defaulted to keep unless the image was constant (not infoimage) where it defaulted to stretch. It now defaults to stretch like all images.
[guilib] remove inconsistent positioning of labels inside containers. <left> now mean left
[guilib] bump xbmc skin version to 5.1.0, still bw compat to 5.0.0
[cosmetic] drop unused function
[cosmetic] drop function definitions that no longer exist
Merge pull request #4613 from jmarshallnz/gui_drop_backcompat

[guilib] Drop back compatibility code and inconsistencies