Convert CommonMethods to Common::Element

In addition to a more descriptive name, this also broke up the various
methods in CommonMethods into more descriptive modules--Positioning,
Visibility and Remove.
Rename Common::Element to Common::UIElement for clarity
Merge branch 'master' into revise_common_methods

Conflicts:
	lib/shoes/background.rb
	lib/shoes/oval.rb
Merge pull request #797 from shoes/revise_common_methods

Convert CommonMethods to Common::Element

Changelog: improvement