[TASK] Introduce form option 'templateFile'

This option gets set automatically if the form is created from Template
[TASK] Add hasOption method to Form
[FEATURE] Implement convention based locations for template icons

Given a template at EXT:extensionKey/Resources/Private/$controller/$templateName.html
If an icon file exists at EXT:extensionKey/Resources/Public/Icons/$controller/$templateName.(png|gif) it will be used as the icon for this template.

Icon can be set manually via option attribute as before.
Merge pull request #687 from coding46/IconNameByConvention

[FEATURE] Icon name by convention