Replace _init() with constructor() where possible

[amezin]

https://gjs.guide/guides/gobject/subclassing.html#subclassing-gobject :

Note that prior to GJS 1.72 (GNOME 42), it was required to override _init() and chain-up with super._init() instead of the standard constructor().

If someone (other than me) decides to work on this: please create a pull request per file/per class. No large PRs touching the whole repository at once.