Support for multiple actions per item

This pull request adds support for having multiple actions for each template item. Depending on which mouse button was used to click on the bar, suitable action will be run.

New tag supported in a template string is ```<buttonX></buttonX>```, where X is a number from 1 to 5. Tag ```<action></action>``` still works and acts the same as ```<button1></button1>```. These tags can be nested, allowing one template item to respond to different buttons. Specifying two or more tags for one button will result in their actions being run at the same time.

Order of tags opened and closed have to be preserved. If a mismatched tag is parsed, it results in a "Could not parse string: " message being printed.

I think that documentation should be updated after merging to reflect these changes. If anything is unclear, please ask.