Added viewport size values to the Editor.
They aren't used in the Desktop version although they are needed for the Mockup in order to achieve resolution independence in all the devices.
For example, let's say I want my Widget to have a width equal to the 20% of the total screen I would set it's preferred width to Editor.getWidth() * 0.2f;
Since we use a virtual viewport in Mockup they replace the traditional Gdx.graphics.getWidth()/Gdx.graphics.getHeight().
Changed Mockup widget to be resolution independent.
Some code cleaning.
Merge branch 'ViewportWidthHeight' of https://github.com/e-ucm/ead into ViewportWidthHeight
Removed width and height constants from Editor, now uses platform-specific size.
Merge branch 'master' of https://github.com/e-ucm/ead into ViewportWidthHeight
Merge pull request #83 from e-ucm/ViewportWidthHeight

Viewport width height and some code cleaning.
Format
Fix in notification of events in model
Remove unused xml
Fix in menu builder
Convenient methods for file widget
Documentation and small fixes to options panel
Added valid state and error message to option; also javadoc
Skin tweaks
Uneeded classes
Added options mechanisms; also constraints
Some strings
Added dialog controller
Added showDialog to views