adding the autocomplete & highlight selected feature for better coding

Submitted by Evel Liu

Assigned to Matthew Brush @matt

Link to original bug (#13597)

Description

Can we improve the mousepad by adding the autocomplete and highlight selected words features？

Generaly, these are a common features for the code editor. it would be great if mousepad can support this 2 features.

For example, when I using perl. I input the word "req" then it would hinting me the "require" keyword. when I double click on the keyword "require", all the instances of the keyword used in the code will be highlighted.

Version: 0.4.0