fix bridge master:slave mapping by-name

Summary:
a slave may be mapped to a master by either the uuid of the master OR
the connection name of the master.

upon widget construction we'll store our current name and if it is the
one referenced by the slave we consider ourselves master of that slave.

also partially unpack the if condition so it's a bit easier on the eyes
with the newly added complexity

Test Plan:
```
nmcli c add type bridge ifname br0 autoconnect yes con-name br0 stp off
nmcli c add type bridge-slave autoconnect yes con-name enp4s0 ifname enp4s0 master br0
```

kcm correctly shows enp4s0 as slave of br0

Reviewers: jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23125