closes #19

Add a configurable welcome video thing

See merge request GNOME/gnome-tour!16

was mostly there for helping test the video feature

600x300px

you should be able to configure the video path at build
time by passing --video_path=absolute_path
which will be played instead of displaying the LOGO

hide the app from the launcher

See merge request GNOME/gnome-tour!20

It doesn't need to be exposed since its a system app, like initial-setup. 

New lockfile

See merge request GNOME/gnome-tour!19

Clean-up edition-related things

See merge request GNOME/gnome-tour!18

assets: lower contrast on Fragments icon

See merge request GNOME/gnome-tour!17

```
Updating aho-corasick v0.7.4 -> v0.7.13
    Updating anyhow v1.0.31 -> v1.0.32
    Updating cc v1.0.37 -> v1.0.58
    Updating cfg-if v0.1.9 -> v0.1.10
    Updating either v1.5.3 -> v1.6.0
    Updating gdk v0.13.0 -> v0.13.1
    Updating gettext-rs v0.4.1 -> v0.4.4
    Updating gettext-sys v0.19.8 -> v0.19.9
    Updating gtk v0.9.0 -> v0.9.1
    Updating libc v0.2.59 -> v0.2.74
    Updating log v0.4.8 -> v0.4.11
    Updating memchr v2.2.1 -> v2.3.3
    Updating pango v0.9.0 -> v0.9.1
    Updating pin-project v0.4.22 -> v0.4.23
    Updating pin-project-internal v0.4.22 -> v0.4.23
    Updating pkg-config v0.3.14 -> v0.3.18
    Updating proc-macro-error v1.0.3 -> v1.0.4
    Updating proc-macro-error-attr v1.0.3 -> v1.0.4
    Updating proc-macro-hack v0.5.11 -> v0.5.18
    Updating proc-macro-nested v0.1.3 -> v0.1.6
    Updating proc-macro2 v1.0.7 -> v1.0.19
    Updating quote v1.0.2 -> v1.0.7
    Updating regex v1.1.9 -> v1.3.9
    Updating regex-syntax v0.6.8 -> v0.6.18
    Updating serde v1.0.114 -> v1.0.115
    Updating syn v1.0.13 -> v1.0.38
    Removing syn-mid v0.5.0
    Updating thread_local v0.3.6 -> v1.0.1
    Removing ucd-util v0.1.3
    Updating unicode-xid v0.2.0 -> v0.2.1
    Removing utf8-ranges v1.0.3
    Updating winapi v0.3.7 -> v0.3.9
```