[processing] JoinAttributes improvement

- allow join not only with vector attribute table but also with geometryless table
- performance and key comparison improvement
- str to unicode fix
[processing] fix join to layer with join (fix #10244)
Merge pull request #1498 from minorua/joinattrs

[processing] JoinAttributes improvement
[FEATURE][composer] Data defined item rotation. Funded by Canton of Neuchâtel, Switzerland