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Fixes #4

Fixes #13 and related to #12

This commit makes it explicit when tabs model url is changing
url of the webview. Without such decoupling, it mobile google search
result was restored as a tab, clicking on search leads to change in url
and resets page back to its older state making it impossible
to a new enter search string

It should always be created using instance()

Creating it seems to take some time, so while it is not yet finished don't try to screenshot it.

* Allows to store isMobile again

Advantages:

* Model information is not duplicated between the qml part (QtQml.Models) and the browserManager::m_tabs
* Changes do not need to be synced with browser manager anymore
* properly testable (and test implemented)
* A lot less logic in the ui (qml part)
* No json inside the ini file

Drawbacks:
* isMobile is not saved to disk anymore to be able to use a simple ini list (doesn't matter much to me, please comment if it does to you, ideally with idea how to store it)

General Changed:
* BrowserManager is now a singleton with a static instance getter, to share one QSettings instance throughout Angelfish

 #This is the commit message #3:

Fixes one warning from QtWebEngine and probably improves performance

In case of conflict in i18n, keep the version of the branch "ours"
To resolve a particular conflict, "git checkout --ours path/to/file.desktop"

In case of conflict in i18n, keep the version of the branch "ours"
To resolve a particular conflict, "git checkout --ours path/to/file.desktop"

Private tabs are now in a separate tab list. Current tab list is
accessible via tabs property allowing to view private and regular
tabs separately. Regular tabs are restored, private are not. On
new tab creation, regular open the homepage while private ones are
set to the blank page.