Add next/previous buttons to Navigate->Find modal bar

[rajeshsegu]

I have been using Brackets for over 3months and I really have some gaps compared to other editors like Eclipse, IntelliJ. One thing that I miss the most if the Next & Previous buttons to navigate across the "Find" match results.

I think this should be part of the core project as its crucially important for many migrating developers like me. Here is a sample screenshots of my implementation.

Case 1: Find has matching results

Case 2: Find has no-results

Case 3: Huge documents in which results are not displayed

Button tool-tips:

Please let me know if this needs any visual improvements.

[njx]

Thanks for working on this! For this bug, we already have the idea on the backlog (https://trello.com/c/MBroELwS), so I'm going to close this issue out. However, I'll link to your pull request from the backlog card so if it gets merged, we'll know it's already done :) Thanks.