Replace 'Ack' plugin by 'Greper'.

Didn't know 'ag' tool before, but after some tests, I am really impress by its speed!

Change-Id: I2df4998e1be70057dc801def72ef20546c973e6a
Changed font to Meslo on GUI

Change-Id: I08fd9fa36b38b4b5fd2f5d10f7670ad8739111a3
Fix mapping conflict between open project tree and re-index helps

Change-Id: I7398e3e23e55970d302ebad242c2f8251ccd7b6e