Add QLR, processing models to browser

[nyalldawson]

Adds some new features to browser:

1. Shows QLR files and allows them to be added to the current project
2. Shows processing models, with context menu allowing the model to be edited directly (double clicking the model executes it)

Motivated because @elpaso has made the browser awesome for 3.0 !

[nirvn]

Hmm, this is all very exciting @elpaso & @nyalldawson . The browser is getting so good it'll soon render the DB Manager mostly irrelevant :)

[gioman]

The browser is getting so good it'll soon render the DB Manager mostly irrelevant :)
Merge state

yes, especially if db manager is almost unmaintained. But what about layers/tables previews and easy creation/management of postgis tables? we should not loose that functionalities.

[nyalldawson]

yes, especially if db manager is almost unmaintained. But what about layers/tables previews and easy creation/management of postgis tables? we should not loose that functionalities.

There's a lot to do still before this could be an option.... maybe for 4.0!

[elpaso]

LGTM, the only question I have if you have considered to drag&drop the model to the canvas instead of (or in addition to) double clicking on it (I'm not saying it would have been a better choice but worths discussing)

[nyalldawson]

@elpaso drag and drop is also implemented here