Currently, the search dialog (search dialog, heading dialog etc) callbacks use indexOf to go to the requested position in the file.

If multiple instances of the search term are present in the file, this prevents us from jumping to anything except the first instance.

For example, consider the following structure

## Project 1
### notes <- instead stops here

## Project 2
### note

Hello, lately I can't successfully vdirsyncer sync my calendars because of a
database is locked error. Synchronization works fine with android's davdroid which talks with the same server.

Your vdirsyncer version: 0.16.4
If applicable, which server software (and which version) you're using: Nextcloud 13
Your Python version: 3.6.4
Your operating system: Arch linux
Relevant debug outpu

Hey shacker, incredible work on this lightweight but powerful todo app! Thank you.

I had one suggestion for future improvements, which is to add a confirm before deleting the task. Keep it up, really appreciate the work.