Couple bug fixes and feature enhancements

[zminjie]

Sorry to group a bunch of things in the pull request because I wrote them as I needed them and forgot to branch...

Enhancements

• ofxDatGuiDropdown
  • added the ability to select an option by name
  • added the ability to reset the options after creation
• ofxDatGuiTemplate
  • added label to width ratio to templates so lwidth is customizable
• ofxDatGui
  • Changed GUIs to acquire focus on mouse over instead of having to manually click

Bug fixes

• ofxDatGuiToggle
  • Fixed toggles still being drawn when set to invisible
• ofxDatGui
  • Fixed components still processing events when set to invisible (you can test this by creating a gui with a footer and hiding the last few components, they'll be placed at the footer location due to layoutGui() and hijack events because they are ordered before the footer)