[Relationships view]Messages no db & empty db

[sam-m888]

WIP idea on what users should be shown initially to provide some guidance and improve usability.

The following messages shows in the empty display area for the [Relationships view]:

• When no database has been created. Displays a message, along with an icon to use the "Family Tree" manager to create one.

(Not done yet)

• When an empty database is loaded and no people are present. Displays a message, along with an icon to use the "ADD a new person" icon from PR https://github.com/gramps-project/gramps/pull/392

[codecov-io]

Codecov Report

Merging #581 into master will decrease coverage by 0.01%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #581      +/-   ##
==========================================
- Coverage   42.16%   42.15%   -0.02%     
==========================================
  Files        1066     1066              
  Lines      142853   142889      +36     
==========================================
  Hits        60231    60231              
- Misses      82622    82658      +36

Impacted Files	Coverage Δ
gramps/plugins/view/relview.py	0% <0%> (ø)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 0ae51c9...f730e25. Read the comment docs.