App: Fix crash when activating search result

[ryonakano]

Fixes #677

• The linked issue causes due to access to Gee.List whose size is 0 at Backend.SynapseSearch.find_actions_for_match (match).get (0), so make sure to check for its size before accessing
• Make sure activating the calculator and converter search items does what their tooltip say

Checklists

• Activating result of calculation (e.g. 2+2 results 4) copies it to clipboard
• Activating result of conversion (e.g. 12kg => g results 12000) copies it to clipboard