Explorer plugin - Everything: custom filters

**Is your feature request related to a problem? Please describe.**

Sometimes when searching, it can be useful to limit search within certain criteria:
- excluding all files that are offline (on a disconnected drive)
- searching within audio-only files (since different file extensions extist)
- searching within files smaller than 10mb

In Everythings GUI, this is possible with **filters**, that utilize **functions**. Currently in 1.5a, **filters are not being exposed** through the SDK, but **functions are**.

Functions look like that: `ext:mp3|txt|jpg size:>10kb size:<5mb online:`
Give it a name and you've got a filter.

**Describe the solution you'd like**

A solution would be to manually implement filters.
Consisting of a name and a string of functions, a filter just has to be prepended to the search string (with an empty space between).

Example:
Name: `audio` - Content: `ext:mp3|flac|opus size:<3mb online:`
Usage: `audio: `

Additionally, one filter should be markable as default, always being applied when omitting its name (unless another filter is used).

**Describe alternatives you've considered**

The alternative is to manually specify functions with every search.