Added PDFGenerator available check to download PDF button.

- Added PDFGenerator available check to download PDF button before running show/hide or enable/disable commands on it. This was causing JS error running actions when button is not in toolbars.
- Refactored code that was running JS commands on toolbar buttons to show/hide or enable/disable them.
Removed report AR Object from session.

Saving miq task id that is being used by render_xxx methods to look up MiqTask record for downloading a timeline report.
Merge pull request #231 from h-kataria/tl_center_tb_actions_fix

Timelines: Fix download PDF button enable/disable, remove report data from session