Remove mode-start and mode-end slots from toolbar

[brandyscarney]

Ionic version: (check one with "x")
[ ] 1.x (For Ionic 1.x issues, please use https://github.com/ionic-team/ionic-v1)
[ ] 2.x
[ ] 3.x
[x] 4.x

Right now we have mode-start and mode-end to recreate the functionality that start and end used to have.

For example:

v3

<ion-toolbar>
  <ion-buttons start>
    <button ion-button (click)="dismiss()">Cancel</button>
  </ion-buttons>

  <ion-title>
    Filter Sessions
  </ion-title>

  <ion-buttons end>
    <button ion-button (click)="applyFilters()" strong>Done</button>
  </ion-buttons>
</ion-toolbar>

v4

<ion-toolbar>
  <ion-buttons slot="mode-start">
    <ion-button (click)="dismiss()">Cancel</ion-button>
  </ion-buttons>

  <ion-title>
    Filter Sessions
  </ion-title>

  <ion-buttons slot="mode-end">
    <ion-button (click)="applyFilters()" strong>Done</ion-button>
  </ion-buttons>
</ion-toolbar>

I'd like to remove these and instead just use start and end, but first I need to verify that this functionality can still easily be added. The old functionality caused a lot of confusion and wasn't intuitive.

More info here: https://github.com/ionic-team/ionic/blob/master/BREAKING.md#attributes-renamed-4

Useful comments:

• <ion-buttons start> not working inside <ion-navbar> #12223 (comment)
• start attribute not working in toolbar #6844 (comment)

[brandyscarney]

Revert

[brandyscarney]

After some further discussion on this we decided to move forward with the following change in naming:

Old Property	New Property	Property Behavior
start	slot="secondary"	Positions element to the left of the content in ios mode, and directly to the right in md mode.
end	slot="primary"	Positions element to the right of the content in ios mode, and to the far right in md mode.
left	slot="start"	Positions to the left of the content in LTR, and to the right in RTL.
right	slot="end"	Positions to the right of the content in LTR, and to the left in RTL.

This table can also be found in the breaking changes document for v4

Here is a diagram to further explain why we went with this naming convention:

[ionitron-bot]

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