user-accounts: user avatar dialog

I started the implementation of the User Avatar Dialog mockup[1]. I marked't as a draft because there are tasks to do.

To do:

• change user avatar popover for a window (figure 1);
• replace crop area dialog for a window (figure 2). Fixed #2515;
• shows check icon, when select picture in provided images (figure 3);
• add more custom pictures and shows check icon (figure 4). For users who are administrators and want to change a photo of another user, it will not be possible to choose a custom image, only the images provided by the system (figure 5), see commit !1869 (9f2ceb1e)

Thank's!

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Refs: [1] https://gitlab.gnome.org/Teams/Design/settings-mockups/-/blob/master/users/user-avatar-dialog.png